> how would one know if AI created something unique or just copied some human's copyrighted work and presented it in a response?
You can't. Similarly, when a human writes original code, we can't be certain that they're not repeating stuff they've seen before too. We don't think of things in terms of license, no human remembers the fast inverse square root function or cocktail sort for the license it had. At no point can I be certain that I'm not unconsciously plagarizing proprietary code from a previous job. Sometimes it's essential (I have to plagiarize "set euo pipefail").
There is indeed some nuance to AI creating supposedly-novel works, but I don't think it's as great as people think. AI makes the process of building a derivative work easier, which is probably frustrating as a license-holder. Many forms of derivative works have legal precedent though, like Wine, DXVK, OpenJDK or the Dolphin emulator - it's an open-source implementation of a proprietary API. You can fly pretty close to the sun without getting burned in many of these cases, as long as you don't violate fair use.